This invention relates to the fabrication of recessed screw heads and, particularly, screw heads of the type which are commonly cold formed on two-blow header machines. These machines typically include a die which rigidly holds a pre-cut length of wire stock while a first punch is impacted against an end of the wire blank to partially form the head and define an "upset". With the upset still held firmly in the die a second finishing punch is brought into axial alignment with the upset and is then impacted axially into the upset bloom of the blank. The finishing punch commonly in use is of one piece, unitary construction having a central forwardly projecting nib shaped to form the recess upon impact. The forward surface of the punch which surrounds the nib defines the shape of the screw head. The one piece punch typically is held rigid as it is impacted into the upset bloom. The use of the one piece unitary finishing punch in the second blow of the two blow heading operation presents a number of difficulties.
Among these difficulties is that the one piece finishing punch has but a relatively short life. It is not uncommon for a punch to become damaged or severly worn after having formed between ten thousand and twenty thousand screw heads. Where heading machines of the type described often have a capacity of up to three hundred screw heads per minute, the punch frequently becomes unusable after only a few hours of operation. This requires interruption of the header on numerous occasions in a single day to enable the punch to be replaced. The down time of the header is sometimes considerable in that even after the punch is replaced, the various parts of the heading machine may have to be realigned and/or readjusted. Further, in typical screw manufacturing factories where many such heading machines are operating simultaneously, it is sometimes difficult to detect when a particular punch on a particular header has become defective and is ready for replacement. Failure to detect a faulty punch in time may result in a high rate of product rejection because the screws formed by the faulty punch will necessarily be of substandard quality.
The above difficulties are amplified when making certain types of screw recesses. For example, where the screw recess is one having a number of grooves and faces formed to receive a specially shaped driver, the punch nib must be formed to define complementary surfaces. By way of example, one such recess and one piece finishing punch therefor is shown in U.S. Pat. No. 3,108,623. The punch nib employed to cold form such a recess necessarily has numerous relatively sharp edges which are easily subject to wear and which form high stress regions which may tend to fracture or deform during the heading process.
The above problems inherent in one piece finishing punches are believed to be due, at least in part, to defects and irregularities in the wire stock from which screws are formed. It is believed that the irregularities may present variable resistance to the cold flow of metal in different parts of the screw head being formed. The existence of irregular, hard regions within the metal of the upset bloom may also cause uneven wear on certain regions of the punch nib which, in turn, may result in increasing the stress imposed on certain parts of the punch nib, resulting in premature failure of the punch. Further, it sometimes occurs that the upset bloom formed by the first blow in the heading operation is asymetrical so that even if the second punch is moved into alignment with the axis of screw blank, the punch nib will not engage the upset bloom symmetrically in its center. This too can impose excessive stresses in the nib which may accelerate early failure.
In addition to the above difficulties, there is evidence that, when using a one piece punch, even where a screw head having an externally satisfactory appearance has been formed, the flow of metal within the screw head often is not uniform but may include hidden cracks and defects which weaken the structure of the screw. Further defects often encountered with screws having a number of substantially radial grooves defining the screw recess is that the critical surfaces and walls of the screw recess often do not conform to the configuration of the corresponding surfaces of the punch nib which form the walls. This is sometimes referred to as "fallaway" and a screw recess having significant fallaway cannot be driven with the same effectiveness as one which has less or no fallaway.
Many efforts have been made to avoid the foregoing difficulties and numerous suggestions have been propesed over a greater number of years. None have succeeded as evidenced by the fact that the one piece finishing punch still is in substantially widespread, universal use.
It has been suggested that many of the foregoing problems of one piece finishing punches could be relieved by employing a punch which could move to follow the fefects in the screw blank or the misalignments of the punch with the screw heads. Typical of such devices are those described in U.S. Pat. Nos. 2,082,085, 2,568,439, 2,643,142 and 2,986,752. In practice, these devices have failed. After a relatively few number of header impacts, they have become jammed and have been unable to shift about to follow the path of least resistance through the upset bloom to conform to metal defects, or other misalignments. Thus, although the proposed solution of mounting the punch for shifting movement has been known in theory for some time, no practical operative device has been developed.
It may be noted tht the recessed screw art has had a number of proposals relating to specifically formed recesses in which the walls of the recess are undercut to more effectively lock the driver to the screw head. One such proposed recess is described in U.S. Pat. No. 2,800,829 to West. That recess, could not be made in a two blow cold-forming process because there was simply no known way to form the undercut wall. In U.S. Pat. No. 2,986,752 to Lovisek a punch and method was described and was stated to set forth a workable method for making the West recess. That punch, however, sufferend from the same defects as the other one piece punches discussed above in that after a few strokes of the header, the punch would bind and would be incapable of shifting and moving about in the manner suggested by the patent.
Also among the difficulties encountered with typical prior art punches is the tendency for the cold-formed screw to stick to the nib of the punch and to be withdrawn from the die as the punch is retracted. This tends to occur more often with shorter screws which cannot be held tightly enough in the die to enable the nib of the punch to be separated from the recess. When this occurs and the punch is operated in the next cycle to form a subsequent recess, the previously formed screw which remains firmly attached to the punch nib can cause failure of the punch nib and can otherwise damage the machine.
It is among the primary objects of my invention to provide a method for forming recesses and a punch therefor which overcomes the foregoing as well as other difficulties described herein.